When reproducing information recorded on a recording medium such as an optical disk, optical-magnetic disk, magnetic disk, or magnetic tape, it is necessary to generate sampling clock signals synchronized with the pulse train which is the reproduced information obtained from the read heads.
Phase locked loop circuits are widely used to generate this type of sampling clock signals. Phase locked loop circuits may be analog circuits or digital circuits. The digital circuits include partially digital circuits which partly include analog components such as oscillator portions, and completely digital circuits which do not include any analog components.
Of these phase locked loop circuits, the completely digital phase locked loop circuits have the advantages that they can be integrated to a very high degree and do not require various types of adjustments. An example of such a phase locked loop circuit is disclosed in U.S. Pat. No. 5,442,315.
However, various problems occur with this type of conventional digital phase locked loop circuit, especially when employed in the playback circuit of a magnetic tape apparatus.
In a magnetic tape apparatus, for example, a phenomenon called “drop out” occurs wherein reproduced information is not attained. This is caused by defects in the magnetic tape medium itself, or the adhesion of dust or dirt to the magnetic heads. When drop out occurs, the phase locked loop circuit slips from a state where the input frequency and phase to be locked are normal and drops out of lock. A conventional phase locked loop circuit does not have a function of detecting this dropping from lock for each channel. Moreover, after the loop drops out of lock because of drop out, even if normal reproduced information is input, the mobile frequency cannot be easily synchronized because the mobile frequency slips with respect to input and the time to recover the locked state is long.
Also, in a magnetic tape apparatus, the magnetic tape travels while in contact with the magnetic heads and therefore the traveling speed of the magnetic tape varies because of variations in the frictional resistance between the magnetic tape and magnetic head due to the capacity of the traveling motor, variations in the exciting current of the traveling motor, variations in tape tension, or the adhesion of dust and dirt. Such variations in the traveling speed are especially large in comparison to the variations in rotation speed of different types of disks.
Variations in traveling speed occur in the same way during writing and reading, but may also occur in mutually opposite directions. In this case, the result is that the traveling speed during writing and the traveling speed during reading vary to extremes and the phase locked loop circuit may not be able to track the variations.
Consequently, while performing reading at the same time as writing, it is necessary to monitor variations in traveling speed continually and when the traveling speed varies in excess of a prescribed range, to execute a process to rewind the magnetic tape once and perform writing once more. However, a conventional digital phase locked loop circuit does not have a function for monitoring variations in the traveling speed of the recording medium.